In 3GPP (3rd Generation Partnership Project) as UMTS (Universal Mobile Telecommunications Systems) SIP (Session Initiation Protocol) is used to establish multimedia sessions or calls e. g. VoIP (Voice over Internet Protocol) sessions.
SIP is generally described e.g. in “SIP: Session Initiation Protocol” by Handley/Schulzrinne/Schooler/Rosenberg; Internet Engineering Task Force SIP WG; internet-draft: ietf-sip-rfc2543bis-02 dated Nov. 24, 2000.
A multimedia session is defined as a set of a multimedia sender and a multimedia receiver whereby data streams flowing from the sender to the receiver. Multimedia sessions include internet multimedia conferences, internet telephone calls and multimedia distribution.
SIP is a request-response control (signalling) protocol for initiating, maintaining and terminating sessions between terminals. SIP is an application level protocol which is used in packet switched environments, e. g. GPRS (General packet radio service) systems, UMTS or packet cable (USA cable modem standard) systems. SIP can be used in any reliable or unreliable protocol including UDP (User Datagram Protocol), SCTP (Stream Control Transmission Protocol) and TCP (Transmission Control Protocol).
SIP uses SIP messages to negotiate between the participants of a SIP session, i.e. between parties or terminals.
SIP, however, is not only used for sessions between two parties but also for group calls, such as conference calls. In a group call all parties can in turn talk or send videos and listen to each other. A group call is used in various activities in which several users participate, e.g. when a whole group must continuously be aware of the progress of events.
Such group calls as well as automatic group call services can be implemented on top of SIP standards, such as the implementation of PMR (Private Mobile Radio) over SIP.
When a terminal registers to a network it is determined whether there are existing or remembered group calls for that party or terminal. If there are such remembered group calls SIP INIVITE messages are sent to the other participants of such a group call in order to set up SIP sessions with each of the other terminals.
However, SIP requires some amount of signalling for each SIP session set-up. For applications where the SIP user agent of a party or terminal is involved in many sessions and/or in many group calls that are typically long lasting, where the user terminal can be frequently powered on and off, and/or network capacity between the terminals' SIP user agents and the network is scarce or needs otherwise to be preserved, the amount of session related signalling as to the known SIP implementations may be too much.